kThemePackVersion comment should mention theme cache flushing.

chrome/browser/themes/browser_theme_pack.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/themes/browser_theme_pack.h"
 
 #include <limits.h>
 #include <stddef.h>
 
 #include <limits>
@@ skipping 30 matching lines @@
 #include "ui/gfx/skia_util.h"
 #include "ui/resources/grit/ui_resources.h"
 
 using content::BrowserThread;
 using extensions::Extension;
 
 namespace {
 
 // Version number of the current theme pack. We just throw out and rebuild
 // theme packs that aren't int-equal to this. Increment this number if you
-// change default theme assets.
+// change default theme assets or otherwise need to flush the theme image cache.

[Evan Stade, 2016/03/17 01:06:51]

actually the "cache" is a bit vague here. Can we say something like "or if you
need themes to recreate their generated images."

[shrike, 2016/03/17 20:59:15]

I'm OK with that, except I think it should explicitly mention that these images
are cached. Please let me know what you think of my edit.

 const int kThemePackVersion = 40;
 
 // IDs that are in the DataPack won't clash with the positive integer
 // uint16_t. kHeaderID should always have the maximum value because we want the
 // "header" to be written last. That way we can detect whether the pack was
 // successfully written and ignore and regenerate if it was only partially
 // written (i.e. chrome crashed on a different thread while writing the pack).
 const int kMaxID = 0x0000FFFF;  // Max unsigned 16-bit int.
 const int kHeaderID = kMaxID - 1;
 const int kTintsID = kMaxID - 2;
@@ skipping 1480 matching lines @@
                                            false,
                                            &bitmap_data)) {
       NOTREACHED() << "Unable to encode theme image for prs_id="
                    << prs_id << " for scale_factor=" << scale_factors_[i];
       break;
     }
     image_memory_[scaled_raw_id] =
         base::RefCountedBytes::TakeVector(&bitmap_data);
   }
 }